In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs) over established radio resource control (RRC) connections. Radio link monitoring (RLM) is a mechanism for a UE to monitor the quality of a downlink (DL) channel of its serving cell for determining if the radio link is good enough to continue transmission. For example, the UE measures cell-specific reference signal (CRS) to detect the downlink radio link quality for the serving cell. The UE also compares the estimated DL quality to thresholds (e.g., QOUT and QIN) for determining if the link between the serving cell and the UE is good enough or not. In addition to RLM, the UE declares radio link failure (RLF) upon the occurrences of physical layer problems based on N310/N311/T310 mechanism, random access problem indication from MAC layer, and indication from RLC layer that the maximum number of retransmission has been reached. Once RLF is detected, the UE gathers and stores RLF information and attempts to restore the RRC connection by performing an RRC reestablishment procedure.
For mobility management in LTE systems, each UE needs to periodically measure the received reference signal power and the qualities of the serving cell and neighbor cells and reports measurement results to its serving eNB for potential handover or cell reselection. Measurements, such as Reference signal received power (RSRP) and/or Reference signal received quality (RSRQ) of an LTE cell, are used to rank among the different cells for the purpose of mobility management. Properly managed handover can prevent loss of connection. In practice, however, handover failure (HOF) often occurs due to various reasons such as UE signaling problems and UE measurement configuration problems. Typically, a radio link failure or handover failure indicates too early handover, too late handover, or handover to a wrong cell. After the RLF/HOF event, the UE will attempt an RRC reestablishment procedure to restore the RRC connection.
When performing RRC reestablishment, the UE releases current RRC configuration and performs cell selection. The prerequisite of a successful RRC reestablishment procedure is that the selected cell for RRC reestablishment has UE context. If the UE fails to restore the RRC connection, then the UE enters RRC idle mode and tries to camp on a cell via a non-access Stratum (NAS) recovery procedure. The UE may indicate the availability of the RLF report to eNB and report the RLF/HOF information to eNB upon request after successful RRC connection reestablishment or RRC connection setup. Based on the RLF report, possible corrective action may be applied by the network to prevent future connection failures.
An LTE-Advanced (LTE-A) system improves spectrum efficiency by utilizing a diverse set of base stations deployed in a heterogeneous network (HetNet) fashion. Using a mixture of macro, pico, femto and relay base stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband user experience. In a heterogeneous network, smarter resource coordination among base stations, better base station selection strategies and more advance techniques for efficient interference management can provide substantial gains in throughput and user experience as compared to a conventional homogeneous network.
In HetNet scenario (e.g., macro-pico deployment), however, it is expected that HOF/RLF rate would increase. For example, HOF/RLF may occur due to imprecise pico cell measurement or not enough time for pico-macro handover. It is thus desirable to improve the connection recovery procedure to reduce outrage time and to reduce data loss during the connection recovery.